Note: although CD21 is the receptor for EBV, it is not necessarily expressed in EBV+ tumors

Pathophysiology

Forms a complex with CD19, CD81 and CD225 in the membrane of mature B cells; complex is often called the B cell coreceptor complex, because CD21 binds to antigens through attached C3d (or iC3b or C3dg) when the membrane IgM binds to the antigen, which results in a greatly enhanced response B cell response to the antigen (Wikipedia)

Besides EBV and HHV8, also binds to breakdown products of complement component C3 (C3d), CD23 (plays a role in IgE synthesis) and possibly gamma interferon

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